Colloidal fumed silica nanoparticles currently are used as fillers in “microfilled” composite dental restorative resins. These particles can increase the hardness and wear resistance of photocured matrix polymers; however, the particles are not radiopaque and cannot be homogeneously dispersed within the matrix resin because of interparticle associations. The resulting coagulation leads to a substantial increase in viscosity and a consequent decrease in composite workability. This places a severe limitation on the practical filler loading in “microfilled” composites.
The loading problem can be partially offset by incorporating prepolymerized organic fillers into the resin in which a relatively high level of colloidal silica is incorporated into highly crosslinked polymeric granules. The workability of the composite resins containing these fillers is maintained, and the cure shrinkage is somewhat reduced. However, the fillers also yield failure prone interfaces and cause a high degree of light scattering, thereby limiting the depth of cure.
Photocurable dental repair materials are needed which are transparent or translucent, radioopaque, have good workabililty, and have good mechanical strength and stability. The use of metal oxides as fillers in such materials results in transparent or translucent, radioopaque materials. However, the hydroxyl groups that tend to form at the surface of metal oxide particles in “protic” environments tend to make the surface of the particles hydrophilic. As a result, the metal oxide particles have difficulty being wetted or adhered to by relatively hydrophobic matrix monomers, such as acrylic monomers, which are non-polar or only weakly polar in nature.
Methods are need to functionalize as many of these hydroxyl groups as possible in order to render the surface of metal oxide particles more hydrophobic.